All Free Biology MCQs with Answers

3691. Which of the following describes conditions in a photosynthesizing cell exposed to high light intensity and low carbon dioxide concentration?

• A. High ATP and NADPH levels, but low Calvin cycle activity
• B. Low ATP and NADPH levels due to fast Calvin cycle activity
• C. Low ATP and NADPH levels due to weak light reactions
• D. High ATP and NADPH use because CO₂ fixation increases

Explanation: Under high light intensity and low CO₂ concentration, photosynthetic cells accumulate ATP and NADPH since the light reactions are active but the Calvin cycle is limited by the shortage of CO₂. Therefore, the correct description is high ATP and NADPH levels with low Calvin cycle activity.

3692. Which of the following is a molecule formed in a metabolic pathway by the equal splitting of a phosphorylated hexose into two halves?

• A. Acetyle coenzyme A
• B. Ribulose-1,5-bisphosphate
• C. Fructose 1, 6-bisphosphate
• D. Triose phosphate

Explanation: Yes, the correct answer is triose phosphate. In the process of glycolysis, a six-carbon sugar called glucose is first converted into a phosphorylated hexose, fructose-1,6-bisphosphate. This compound is then split equally into two three-carbon molecules-glyceraldehyde-3-phosphate (G3P) and dihydroxyacetone phosphate (DHAP). Both of these are known as triose phosphates. This step marks the transition from a six-carbon to a three-carbon stage in glycolysis, allowing energy extraction to continue in later steps of the metabolic pathway.

3693. What is the function of molecular oxygen in cellular respiration?

• A. Oxygen is the final electron acceptor in the electron transport chain
• B. Oxygen breaks down glucose during glycolysis
• C. Oxygen is used to directly produce ATP in the Krebs cycle
• D. Oxygen acts as an enzyme to speed up cellular reactions

Explanation: The correct answer is A-Oxygen is the final electron acceptor in the electron transport chain. Molecular oxygen's key role in cellular respiration is to accept electrons and hydrogen ions at the end of the electron transport chain to form water. This step is vital because it maintains the flow of electrons through the chain, allowing the cell to continue generating large amounts of ATP through oxidative phosphorylation. Oxygen is not involved in glycolysis, does not directly participate in the Krebs cycle, and is not an enzyme. Without oxygen, the electron transport chain stops, and cells must rely on much less efficient anaerobic pathways for energy production.

3694. The reaction that occurs in the thylakoid interior space:

• A. Photolysis
• B. ATP synthesis
• C. Transport of electrons
• D. Dark reaction

Explanation: The correct answer is photolysis, which occurs in the thylakoid interior space during the light-dependent reactions of photosynthesis. Water molecules are split into oxygen, electrons, and protons by the energy from light, a critical step in oxygen generation and energy-rich molecule production. ATP synthesis is closely linked to proton flow through ATP synthase in the thylakoid membrane, not the interior. Transport of electrons occurs across the thylakoid membrane, not specifically in the interior space. Dark reactions, or the Calvin cycle, happen in the chloroplast stroma, independent of light, and are not a thylakoid interior process.

3695. The wavelengths of light least absorbed by carotenes are

• A. Blue to green
• B. Red to orange
• C. Yellow to orange
• D. Yellow to red

Explanation: Carotenes are pigments that appear red to orange because they reflect these colors of light and do not absorb them efficiently. Instead, they absorb light in the blue-violet spectrum. Therefore, the red to orange wavelengths are least absorbed by carotenes, making Option B correct. Options A, C, and D are incorrect because they either focus on wavelengths that carotenes absorb or misrepresent the range of reflection.

3696. The oxidation of which of the following will produce FADH₂?

• A. Malate
• B. Fumarate
• C. FAD
• D. Succinate

Explanation: The correct answer is succinate. In the citric acid cycle, succinate is oxidized to fumarate by the enzyme succinate dehydrogenase. During this reaction, FAD is reduced to FADH₂, which is essential for the electron transport chain and ATP production. Malate is involved in a different step, converting to oxaloacetate and producing NADH instead. Fumarate is a product of succinate oxidation and does not directly produce FADH₂.. FAD is a coenzyme, not a substrate, and is reduced during succinate oxidation, not oxidized itself.

3697. Light energy is converted into chemical energy through the formation of:

• A. NADPH
• B. ATP an NADPH2
• C. ADP
• D. RuBP

Explanation: During the light-dependent reactions of photosynthesis, light energy is captured and converted into chemical energy in the form of ATP and NADPH. These molecules are essential for the subsequent light-independent reactions (Calvin cycle), where they drive the synthesis of glucose. Option B is correct because it includes both ATP and NADPH, which are the products of the light-dependent reactions. Option A is incorrect because it only mentions NADPH, ignoring the role of ATP. Option C is incorrect because ADP is a precursor to ATP and not a final product of the light-dependent reactions. Option D is incorrect because RuBP is not involved in the light-dependent reactions but rather in the Calvin cycle, where it acts as a carbon dioxide acceptor.

3698. Stroma is the ground matrix of:

• A. Lysosomes
• B. Ribosomes
• C. Oxysomes
• D. Chloroplast

Explanation: The correct answer is chloroplast. The stroma is the ground matrix of a chloroplast, which is crucial for the photosynthesis process. It is the site for the Calvin cycle, where carbon fixation takes place. The other options are incorrect: Lysosomes are involved in waste processing, ribosomes are involved in protein synthesis, and 'oxysomes' is not a recognized term in cellular biology.

3699. At which wavelength of light is photosynthesis maximum?

• A. Red light
• B. Blue light
• C. Green light
• D. Far-red light

Explanation: Blue light around 450 nanometers helps plants grow strong leaves and stems, playing a good role in photosynthesis but not reaching its highest level. Green light near 550 nanometers is mostly reflected, which is why plants look green, and it contributes the least to photosynthesis. Red light around 680 nanometers, however, is the most effective for photosynthesis, as it is absorbed best by chlorophyll and powers the light reactions that give plants their energy. Far-red light near 750 nanometers supports some growth functions but has a weaker effect on photosynthesis compared to red light.

3700. The source of protons within the chloroplasts is

• A. Water
• B. Carbon dioxide
• C. Excited chlorophyll molecules
• D. Rubisco

Explanation: In the light-dependent reactions of photosynthesis, which occur in the thylakoid membranes of chloroplasts, water is split in a process known as photolysis. This process generates protons (H⁺ ions), electrons, and oxygen gas. The protons contribute to the proton gradient used by ATP synthase to produce ATP, a crucial energy carrier in the cell. Therefore, water is the source of protons in this context. Carbon dioxide is involved in the Calvin cycle and not in generating protons during the light-dependent reactions. Excited chlorophyll molecules play a role in capturing light energy and initiating electron transport but do not directly release protons. Rubisco is an enzyme that functions in the Calvin cycle, not in the light-dependent reactions where proton generation occurs.